The beam attenuation coefficient and its spectra. (also known as beam-c or extinction coefficient ). Emmanuel Boss, U. of Maine

Transcription

1 The beam attenuation coefficient and its spectra (also known as beam-c or extinction coefficient ). Emmanuel Boss, U. of Maine

2 Review: IOP Theory Collin s lecture 2: F o F t Incident Radiant Flux Transmitted Radiant Flux Roesler and Boss, 2008 Attenuation Collimated Monochromatic

3 c = fractional loss of light per unit distance Beam Attenuation Measurement Theory F o F b F a F t c Dx = - DF/F x x ò 0 c dx = -ò 0 df/f c(x-0) = -[ ln(f x )-ln(f 0 )] c x = -[ ln(f t )-ln(f o )] c x = - ln(f t /F o ) Roesler and Boss, 2008 Dx c (m -1 ) = (-1/x) ln(f t /F o )

4 Beam Attenuation Measurement Reality c = (-1/x) ln(f t /F o ) source F a F b detector F o x Detected flux (F t ) measurement must exclude scattered flux Roesler and Boss, 2008 F t To get a signal detector has finite acceptance angle some forward scattered light is collected.

5 Beam attenuation measurement (single wavelength) Advantages: Well defined optical quantity (for a given acceptance angle). No need to correct for absorption or scattering along the path (unlike the VSF and a). Pathlength matters- assumes single scattering regime. Not dependent on polarization properties. First commercial inherent optical quantity measured (O(1980)) ß long history.

6 Theoretical beam Attenuation: Mie theory (O, homogenous) for phytoplankton and sediments: c ext = attenuation of a single partilce c ext = attenuation of a single partilce n= i Q c = Optical/Geometric cross-section n= i Q c = Optical/Geometric cross-section c = attenuation/volume D [ m] c = attenuation/volume D [ m]

7 Theoretical beam Attenuation: Like all IOP, c p is dependent on size and composition: Particle specific beamattenuation, Beam-c/volume dependence on: Size. n=1.05 Index of refraction. Absorption. Boss et al., 2001

8 c p is sensitive to the wavelength of measurement: The instrumental filter is size dependent: Particle size where maximum beam-c occurs changes by ~ 2µm between blue to red wavelengths. Magnitude and width of maximum change with l.

9 Single wavelength beam attenuation and biogeochemistry: Found to correlate well with: Total suspended mass Particulate organic carbon Particulate volume Phytoplankton pigments in area where growth irradiance is stable.

10 Good correlation with total particle volume, and particulate organic carbon. Peterson (1977) Bishop (1999)

11 But, there is variability in attenuation/mass between studies: What may cause this trend? Hill et al., 2011

12 Particulate Beam Attenuation spectrum Advantages: Well defined optical quantity (for a given acceptance angle). No need to correct for absorption or scattering along the path (unlike the VSF and a). Not dependent on polarization state of source. Available commercially since 1994.

13 Beam Attenuation spectrum Advantage: Simple spectral shape. Little effect due to absorption bands (light that is absorbed is not scattered). Barnard et al., 1998, JGR Boss et al., 2001, JGR Tara Oceans, 40,000 1km 2 spectra

14 An aside: C( λ) C w ( λ) ( ) C w ( 490) C 490 = λ Tara Oceans: Corollary: 1. CDOM contribution relatively constant. 2. PSD does not change (we will get there soon). C( λ) C w ( λ) λ γ

15 Another advantage: c p (l) is sensitive to size 1 n=1.15 normalized c p [nm] The instrumental filter is size dependent: Particle size where maximum occurs changes by ~ 2 between blue to red wavelengths. Magnitude and width of maximum change with l.

16 Beam-c and Particle size distribution (PSD): If a power-law ( Junge-like ) PSD function: N(D)dD=N o (D/D o ) -x Is often used to described oceanic PSDs. How does N(D) looks?

17 N(D)dD=N o (D/D o ) -x Units? Typically, 2.5<x<5 Most frequently 3.5<x<4 Kitchen (1977) Zaneveld and Pak, 1979, off Oregon coast

18 Beam-c and PSD relation: Mie Theory (homogenous spheres): Volz (1954): For non-absorbing particles of the same n and an hyperbolic distribution from D min =0 to D max =, N(D)=N o (D/D o ) -x æ l ( l) = ( l ) ç, x = g 3 c p c p ö -g 0 ç l + 0 è ø à expect a relation between attenuation spectrum and PSD.

19 Example: particles distribution in the bottom boundary layer In BBL we expect that concentration and PSD will co-vary because particle settling is size dependent (w s ~D 2 ).

20 Observations: bottom boundary layer Boss et al., 2001 Particulate attenuation (c p ) and its spectral slope (g) are inversely related in the BBL.

21 Measured PSD exponent Measured c p exponent N=3000 N=64 Boss et al., 2001

22 Observations: Both x and g decrease monotonically with decreasing attenuation. Theoretical and observed relationship between x and g are within 30% of x, despite the potentially large error bars associated with the sampling methods. Better agreement modified theory: g ³ 0 in observation for x<3. Supports the use of g as a tool to estimate the PSD slope. In the least, it describes the changes in the mean particle diameter (proportion of big vs. small).

23 Particulate attenuation spectral slope as a tool to study particle composition and species succession: Roesler and Boss, 2008

24 Beam-c issues: acceptance angle. Instrument Acceptance angle (in-water) AC LISST-B LISST-Floc Path-length 10cm 5cm 5cm beam attenuation [m-1] Jerlov, 1976: less than 5% of scattering in first 1. Petzold, 1972: up to ~30% of scattering in first /04 09/05 09/06 09/07 09/08 09/09 09/10 09/11 09/12 09/04 09/05 09/06 09/07 09/08 09/ Date (GMT) 09/10 09/11 09/12 ratio [unitless] frequency Another issue: Turbulence. (Bogucki et al., 1988) c ac9 (676)/c lisst-b (670) p pg c ac9 (676)/c lisst-f (670) p pg OASIS: Boss & Slade, unpublished c lisst-b (670)/c lisst-f (670) pg pg

25 Handling and aggregates: Aggregates: Boss et al., 2009, Slade et al., 2010, 2011 For particles with D>>l: When scattering centers are far enough, IOPs are additive. Optical properties µ cross-sectional area, additive Depends on aggregate packaging ( fractal dimension). Spectral dependence of scattering µ l 0

26 Beam attenuation of single particles vs. aggregates x=pd/l r=2pd/l(n-1) Stemmann and Boss, 2012 F-solid fraction n part =1.15 n part =1.05

27 Effect of aggregation on mass specific attenuation lab experiment: Slade et al., 2011

28 Global statistics of spectral shape: 57,000miles

29 Global statistics of spectral shape: Boss et al., 2013

30 Before I summarize your role: Beam attenuation is measured on many vessels. c=-ln{(vsig-vdark)/(vref-vdark)}/0.25m What calibration do they use? How do they correct when they get negative data at depth? Your role: Measure Vdark Using a flow-sleeve, measure Vref

31 Summary: Beam attenuation is a robust IOP. Beam-attenuation has a long history. If I had to do a single optical measurement, it would be c(660). Why? Relationship between spectral c p and PSD provide tool to track changes in community composition and sediment dynamics.

32 Selected references: Measurement issues: Pegau, W. S., J. R. V. Zaneveld, and K. J. Voss (1995), Toward closure of the inherent optical properties of natural waters, J. Geophys. Res. 100, 13,193 13,199. Voss, K. J., and R. W. Austin (1993), Beam-attenuation measurements error due to small-angle scattering acceptance, J. Atmos. Oceanic Technol., 10, Relationship to biogeochemistry: Behrenfeld, M.J. and E. Boss, Beam attenuation and chlorophyll concentration as alternative optical indices of phytoplankton biomass. Journal of Marine Research, Vol. 64, pp Bishop, J.K.B., Transmissometer measurements of POC. Deep-Sea Research I 46, Boss, E., M. S. Twardowski, and S. Herring Shape of the particulate beam attenuation spectrum and its inversion to obtain the shape of the particulate size distribution. Appl. Opt. 40: Gardner, W. D., M. J. Richardson, C. A. Carlson, D. Hansell and A.V. Mishonov Determining true particulate organic carbon: bottles, pumps and methodologies. Deep-Sea Res. II., 42, Kitchen, J. and J. R. Zaneveld On the noncorrelation of the vertical structure of light scattering and chlorophyll a in case I waters. J. Geophys. Res., 95, 20,237 20,246. Roesler, C.S. and E. Boss (2008). "In situ measurement of inherent optical properties and potential for harmful algal bloom detection and coastal ecosystem observations" in Real-time Coastal Observing Systems for Marine Ecosystem Dynamics and Harmful Algal Blooms: Theory, Instrumentation and Modelling, Eds. M. Babin, C.S. Roesler and J.J. Cullen. Spinrad, R. W., J. R. V. Zaneveld, and J. C. Kitchen, A study of the optical characteristics of the suspended particles in the benthic nephloid layer of the Scotian shelf, J. Geophys. Res., 88, ,1983. Global distribution: Barnard, A. H., A. H., W. S. Pegau, and J. R. V. Zaneveld Global relationships of the inherent optical properties of the oceans. J. Geophys. Res. 103: Boss, E., M. Picheral, T. Leeuw, A. Chase, E. Karsenti, G. Gorsky, L. Taylor, W. Slade, J. Ras, and H. Claustre, The characteristics of particulate absorption, scattering and attenuation coefficients in the surface ocean; Contribution of the Tara Oceans expedition. Methods in Oceanography,